It is the throne of Mulcarn, and the place where he died. It is in Erebus, which is the setting for FfH2. But you probably figured that out yourself.

It is not at a north or south pole, like on earth. I always imagined it as being the centre of Erebus, with all the continents radiating out from it. Magister Cultuum or Kael could probably give you a precise geographical fix

It is not only the place where mulcarn was killed, but he place where he killed Sucellus, god of nature. It will appear in random areas on maps in FFH2. There is a full forum on the true map of erebus if you can find it. Guys there are attempting to place everything on the true map of erebus, including the races, in their correct possitions. In FFH: AOI, Letum Frigus is slightly south of the top left corner.
Hope this helps.

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The Fall From Heaven mod is downloadable and it is a ton of fun. Much better than Age of Ice (even though Age of Ice is good fun as a scenario too!).

In the Age of Ice scenario, though, Letum Frigus is pretty much due West of your cities and just a few tiles north. It's further west than you might expect since it's kind of in the middle of nowhere on the map.

The Ice cities are in the Northwest corner and Letum Frigus is somewhat south of them.

I don't believe that Letum Frigus was where Mulcarn killed Sucellus, but it probably wasn't too far away. It is rather the place where Mulcarn entered Erebus the first time, and a small piece of his vault that remains in this world. Sucellus was killed close to where Taranis was held on the AoI map. Mulcarnw ould not have been able to defeat Sucellus had not the god of nature been distracted by his attempt to kill Taranis, whose unchanging nature made it impossible for him to be hurt (at least by the god of nature, whose attacks would by his nature cause gradual changes/injury rather than destroying all at once; Bhall probably could have killed Taranis with one hit) and allowed him to keep the god occupied even though he had no hope of actually winning.

The name Letum Frigus, meaning "Cold Ruin," was not applied to the place until Auric Ulvin found it (although it is anachronistically called that in AoI). During the Age of Ice, it was not a ruin at all, but a grand palace. I believe that when Mulcarn ascended he brought a part of his vault with him, specifically the peak Mount Mulyr (I'm not on my computer right now, and think I may have misspelled that) where his palace was located. After Mulcarn was slain, the palace began to fall apart, considering that it was made mostly of ice. The power of winter remained strong here, but it was enough to keep the palace completely frozen and so it left a collapsed mountain filled with empty caverns instead of grandly decorated halls of crystal.

Erebus is a flat, infinite plain, so it has no poles and can't really have a center either. All we really know about Letum Frigus's location is that it is near the lands where the Illians live, but that the Illians have to much fear and reverence for the spot to enter unless specifically told to by their god.